xfce-build: Retain html docs in /apidocs

Summary: This MR adds an apidocs folder to the repo which holds an index.html with a hardcoded list of components that have API documentation. It then copies all the html docs from the components built in the xfce/xfce-build Docker image to the apidocs folder on the runner so it can then be archived as job artifacts. In a second step we can later (periodically) download these artifacts and publish them on a webserver.

The added size is 16MB over xfce/xfce-build without the html docs.